1. Field of the Invention
The present invention relates to a driving system for a fan with a higher maximum static pressure and a method of driving the fan.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2002-61596 discloses the structure of a centrifugal fan as an example of a fan that includes an impeller and a motor rotating the impeller, and that produces a maximum static pressure when the motor rotates the impeller at a maximum speed and that produces a maximum amount of air flow when the motor rotates the impeller at a minimum speed. Japanese Unexamined Patent Application Publication No. 2002-112570 discloses a driving system for a brushless fan motor including a centrifugal fan and a method of driving the brushless fan motor. FIG. 6 shows an example of the circuit of a conventional driving system for a centrifugal fan. The conventional driving system includes a drive signal generating circuit DSC that generates drive signals S1 to S4, and a motor driving circuit MDC that supplies a motor current to a brushless motor in accordance with the drive signals S1 to S4. The motor driving circuit MDC comprises a bridge circuit including transistors (semiconductor switches) SW1 to SW4, regenerative diodes D1 to D4, and a snubber capacitor C. The drive signal generating circuit DSC detects the rotor position on the basis of an output from a magnetic sensor H of a Hall element that detects magnetism of a plurality of permanent magnets provided in a rotor of the brushless motor, and provides the drive signals S1 to S4 with phase differences shown in FIG. 7 to the respective bases of the transistors SW1 to SW4 in accordance with the detected rotor position. The motor driving circuit MDC alternately brings a pair of the transistors SW1 and SW4 and a pair of the transistors SW2 and SW3 into conduction to cause an AC motor current to flow through excitation windings W in order to drive the brushless motor.
As shown in FIG. 8, a fan such as a centrifugal fan has an operational characteristic such that the rotational speed V increases as the static pressure approaches the maximum static pressure MP, and such that the motor current decreases as the static pressure approaches the maximum static pressure MP. FIGS. 9A and 9B show the waveform of the motor current when the conventional driving system generates a maximum amount of air flow and the waveform of the motor current when the conventional driving system generates a maximum static pressure, respectively, corresponding to operating regions I to IV of FIG. 7. As shown in FIG. 9B, when the maximum static pressure is produced, the motor current is decreased, and therefore the torque is also decreased. Consequently, in order to increase the maximum static pressure MP without changing the maximum amount of air flow MQ and power consumption, it is required to increase the motor current supplied to the excitation windings W of the motor at the time of the maximum static pressure MP to increase the torque to secure the required rotational speed.
If the excitation windings W with specifications required to obtain a rotational speed required for the maximum static pressure MP are used with the conventional driving system shown in FIG. 6, however, the rotational speed may become excessive at the time of the maximum amount of air flow MQ to consume excessive power.